Desktop and Other Problems

I turned on my computer, and when I got to the login page it showed a warning message saying my registration info was unsufficient and one of my local files was damaged. I couldn’t log onto my windows account because of this. It wasn’t an incorrect password, I’m 100% sure, it’s something else. This morning however was different. When I got in, eventually, my whole desktop was practically gone. All that’s left is Adobe Reader, Recycle Bin and MSN. I cannot drag any icons or set any as shortcuts. Sometimes I can’t even use anything. Any suggestions on what to do?